This invention relates generally to reducing noise in Photodiode Arrays (PDA), and in the preferred embodiment, to improving the performance of a spectrometer which uses a photodiode array detector. In such a spectrometer, light of different wavelengths is focused on different elements, or pixels, of the PDA. Each pixel consists of a photodiode and an associated storage capacitor which may be the junction capacitance of the photodiode or a physical capacitor incorporated into the PDA. The capacitor is charged to a reference voltage and then partially discharged as the photodiode conducts photo current in response to the light signal. A number of Field Effect Transistor (FET) transfer switches are coupled in series between each pixel and a charge amplifier. Each pixel may be read out in succession by sequentially turning on, therefore, closing, the associated transfer switch. This action also enables the amplifier to recharge the associated capacitor to the reference voltage. The amount of charge required to bring the capacitors up to the reference voltage is defined as the image signal charge and is proportional to the intensity of the light or image incident on the photodiode.
Sensitivity, a measurement of the minimum amount of the image signal or light signal that can be detected, is an important feature of spectrometers used in atomic emission detectors. To monitor the signal of interest, a PDA is used to convert optical emissions into an electronic signal. During operation, electronic noise generated by the PDA can obscure weak signals. The invention reduces this noise, and thereby improves the overall sensitivity of the spectrometer or other apparatus employing the invention.
FIG. 1 shows a conventional PDA 5 connected to a charge amplifier 20. One side of each photodiode capacitor pair is coupled to a common node 6. Serial readout of a charge voltage across each photodiode 13 in the PDA 5 is accomplished by means of a digital shift register (not shown) coupled to the transfer switches 16 (Q.sub.1 -Q.sub.n). Operation of this circuit is as follows:
1. After the previous readout cycle is completed, each pixel capacitor 14 (C.sub.1 -C.sub.n) is charged to a reference voltage V.sub.d. Such charging is carried out by the action of the operational amplifier 23 as the reference voltage is maintained between its non-inverting input and the common node of the photodiode array.
2. After the pixel capacitors 14 have been charged, the transfer switch 16 is opened, so that the photodiode 13 and pixel capacitor 14 are disconnected from the operational amplifier 23 for a specified period of time. An optical or light signal 8 is converted to an image signal by the associated photodiode and is integrated on pixel capacitors C.sub.1. . . C.sub.n, thus discharging each capacitor by an amount of charge representative of the intensity of the optical or light signal 8. The amount of charge removed is defined as the "image signal charge". The term "image signal" is not meant to limit the meaning of "signal" to something corresponding to optical signals which are pictorial in nature. Rather, "image signal" denotes a signal which is a representation of, and is generally proportional to, the intensity of the light of the corresponding optical signal. This optical signal can be either pictorial, as in a camera, or spectral, as in a spectrometer, or of a different nature. The amount of charge removed is defined as the "image signal charge".
3. Transfer switches 16 (Q.sub.1 -Q.sub.n) are employed for successively recharging capacitors 14 (C.sub.1 -C.sub.n) by transferring the image signal charge to the inverting input node 99 of the operational amplifier. The operational amplifier acts to keep the voltage on the inverting input node 99 equal to the voltage on the non-inverting input, by changing the voltage on the output node 97 until the input node 99 equals the voltage on the non-inverting input. In this circuit, the non-inverting input, and therefore the inverting input, are equal to ground. Thus, the inverting input is commonly referred to as virtual ground. At this point, all of the of the image signal charge is held on the feedback capacitor 22 (C.sub.f), and none of it is stored on capacitor 24 (C.sub.a), and the voltage at the output node 97 is proportional to the image signal charge. Each pixel may be successively read in this fashion. After reading each pixel, field effect transistor reset switch 21 (Q.sub.f) is closed to short out capacitor 22 (C.sub.f), then opened to accept the charge from the next pixel.
4. Other circuitry (not shown) reads the value of the image charge signal and stores each reading in a computer memory for processing.
During the reset operation of the operational amplifier (step 3), the closed loop bandwidth of the integrator amplifier 23 is increased significantly. This results in increased noise charge fluctuations at the inverting input node 99. These fluctuations are due to the input voltage noise of the amplifier as well as thermal noise generated by the reset switch 21. At the instant the reset switch is opened, whatever noise voltage happens to be present on the inverting input node 99 is "frozen" on capacitor 22 (C.sub.f).
One prior art technique for reducing this noise is called "correlated double sampling" and employs the analog subtraction circuit 25 shown in FIG. 2 and incorporated into FIG. 1. This circuit consists of a series capacitor 26 and a switch 27 that connects the output end of the capacitor to ground. Switch 27 is turned on just after the integrator amplifier 23 reset operation, and turned off just before the next pixel is connected to the integrator amplifier 23 input. This causes the sum of the amplifier input noise and the thermal noise charges to be stored on capacitor 26, thereby subtracting this term from the output signal. As illustrated in FIG. 1, this circuit is coupled to the output of the charge amplifier 20. However, such an analog subtract circuit is not required for the detection of light signals in a photodiode array, but it does enhance performance.
The FET transfer switch 16 (Q.sub.1) is another source of thermal noise. This noise is sometimes even greater than the reset switch noise. Unfortunately, the correlated double sampling subtraction technique does not address this problem and overall sensitivity of any device employing the PDA is impaired.
In particular, the switching action of the field effect transistor transfer switch in the integration-readout process causes a noise term, called "kTC" noise, to be unfortunately added to each photodiode measurement. This "kTC" noise is a type of thermal noise which is caused by random motion of electrons in some electronic devices, and Field Effect Transistors in particular, and is associated with resetting the photodiode capacitance to a fixed voltage. The term kTC stems from Boltzman's constant "k", the temperature "T" and the capacitance "C". In actuality, this thermal noise charge corresponds to the square root of k*T*C which is technically defined as the Root Mean Square (RMS) noise charge. However, this noise charge will hereinafter be identified as "kTC noise". When PDA's are operated at low light levels, this kTC noise is often the largest noise term. It is the purpose of this invention to eliminate this kTC noise.